A noble metal fine particle supported catalyst in which noble metal fine particles (for example, noble metal colloids described in Patent Literature 1 and Nonpatent Literature 1) that are finely made and each have a particle diameter of about several nanometers are supported on a ceramic material, a carbon material, a metal material or an organic polymer material is used for treating a fluid such as exhaust gas and wastewater.
For example, a conductive catalyst obtained by allowing noble metal fine particles to be supported on carbon powder that is a carbon material is used as an electrode material for a fuel cell. A catalyst obtained by allowing noble metal fine particles to be supported on a ceramic material is used as a exhaust gas purifying catalyst for decomposing components (such as hydrocarbon (HC), carbon monoxide (CO), and a nitrogen oxide (NOx)) contained in high temperature exhaust gas emitted from an internal combustion engine such as an automobile engine, or as a reforming catalyst for generating hydrogen from a fuel, such as alcohol, inside a reformer of a fuel cell. There is also a case where a noble metal fine particle supported catalyst is used as a decomposing catalyst for decomposing environmental toxic substances contained in industrial wastewater, etc.
In a noble metal fine particle supported catalyst, a contact reaction in which a reaction proceeds on surfaces of noble metal fine particles is accelerated. Thus, the catalytic activity of the noble metal fine particle supported catalyst is enhanced in proportion to the surface area of the noble metal fine particles. Therefore, in order to enhance the catalytic activity of the noble metal fine particle supported catalyst, it is effective to use noble metal fine particles that are finely made and have a uniform particle diameter by controlling the particle diameter of the noble metal fine particles. That is, when noble metal fine particles each having a particle diameter of about several nanometers are supported on a surface of a substrate while the particles are dispersed uniformly, the surface area of the noble metal fine particles is increased, and thereby a noble metal fine particle supported catalyst with high catalytic activity can be obtained.
When the substrate has a porous structure, noble metal fine particles can be supported within the substrate (that is, on surfaces of pores in the porous structure) and on a surface of the substrate (that is, an outer surface of the porous structure). Use of a noble metal fine particle supported catalyst having such a porous structure (noble metal fine particle supported porous catalyst) makes it possible to increase further the surface area of the catalyst that contributes to the contact reaction. As the method for producing the noble metal fine particle supported porous catalyst, a common method is known. For example, methods are known in which a porous membrane formed on a substrate is immersed in a noble metal raw material solution, and then noble metal is precipitated (Patent Literatures 2 and 3). Methods are also known in which granular porous supports on which noble metal is supported is mixed with a solvent to make a slurry, and this slurry is applied and deposited on a substrate (Patent Literatures 4 and 5).
There is also known a case in which for the purpose of enhancing the function of a titanium oxide photocatalyst, titanium oxide fine particles on which noble metal fine particles are supported are further supported on a substrate (Patent Literature 6). In addition, there is also known a method in which for the purpose of enhancing the state of noble metal fine particles being dispersed, noble metal fine particles such as Pt particles, powdery supports composed of a metal oxide such as Al2O3, and an inorganic binder such as a silica sol are mixed together, and a resultant mixture is applied to a substrate, the applied coating is dried and sintered to form a coating membrane on the substrate (Patent Literature 7). There is also known a method in which, for the same purpose, a noble metal colloid (noble metal fine particles) prepared using a reversed micelle process, a metal hydroxide prepared using a reversed micelle process, and a hydrolysate of metal alkoxide are mixed together and a resultant mixture is sintered (Patent Literature 8).